1. Field of the Invention
The present invention relates to a controller for controlling wire cut electrical discharge machines to improve their machining accuracy in reentrant angular corners.
2. Description of the Related Art
In machining using a wire cut electrical discharge machine, if the wire electrode (hereinafter referred to simply as the wire) follows the unaltered route specified by a machining program, the resulting contour (machined route) of the workpiece deviates from the specified route.
This fact, which is well known in the art, is mainly due to the non-negligible radius R of the wire and the discharge gap Δgp. More specifically, the wire route differs from the machining route by an amount equal to the wire radius R plus the discharge gap Δgp, and the dimensions of the machined workpiece are reduced accordingly.
A basic approach taken to avoid this problem is to create an offset machining route by offsetting the route specified by the machining program by an amount equivalent to the wire radius R plus the discharge gap Δgp and move the wire along the offset machining route. Multi-pass machining techniques in which this idea of wire offset is adopted are also known.
Multi-pass machining is a technique in which the entire machining process for obtaining the intended contour line (machined route) is divided into a plurality of machining passes with contour lines (machined routes) that progressively approach the final contour line (machined route), the machining route in each pass being selected so that the contour line obtained in the final machining pass reaches matches the intended contour line as closely as possible. In theory, the problem of the deviation of the actual machined contour from the machining route specified by the machining program can be solved by gradually reducing the offset from one pass to the next (gradually approaching or reaching R+Δgp), as if peeling off a succession of skins.
If this technique is applied to the machining of reentrant angular corners, however, it becomes difficult to achieve high machining accuracy. A reentrant angular corner is one type of corner shape created in wire cut electrical discharge machining. The various types of inside and outside corner shapes created by wire cut electrical discharge machining include arcs, right angles, acute angles, and obtuse angles; the type which will be referred to hereinafter as a reentrant angular corner is an inside corner formed by two intersecting straight blocks,
The reason why multi-pass machining has difficulty reaching high accuracy in a reentrant angular corner is that due to the offset, as the corner angle decreases, the machining depth increases, and in the finishing process (the final machining pass or the final few machining cycles) in multi-pass machining a very low discharge energy is usually used to obtain a good surface finish, as if peeling a thin skin. The machining depth in the finishing of a reentrant angular corner may therefore exceed the machining capacity of the discharge energy, in which case the wire and the workpiece will become short-circuited, resulting in erratic machining and poor accuracy.
A typical control technique for avoiding such short-circuiting reduces the machining speed in reentrant angular corners, but it is difficult to determine the exact speed needed because there are several factors involved, including the corner angle and electrical conditions. If the speed is not reduced enough, short-circuiting will occur; if the speed is reduced too much, the electrical discharge will be over-concentrated and the workpiece will be over-machined. Attempts have been made to solve these problems by providing several parameters that can be set to address various assumed situations, but this approach is unacceptable in practice because as the number of assumed situations is increased to improve controllability, a huge amount of time and effort is required to determine the correct parameter settings.
The above problem does not occur in reentrant circular arc corners, because the machining routes are concentric, so the machining depth is no greater than in the machining of straight sections, even if the offset is different in each machining pass.
Known documents discussing the above problem in multi-pass machining of reentrant angular corners include the following, which are listed below with brief outlines of their contents.
(1) Japanese Patent Application Laid-Open No. 59-115125 discloses a method for correcting a wire-cut shape by adding circular arc routes in machined reentrant angular corners. In the wire-cut shape correction method disclosed in this patent document, when the machining routes specified by two instruction blocks in the basic program intersect to form a reentrant angular corner, a circular arc machining route centered at the block intersection and having a radius equivalent to the offset of the wire electrode from the machining route is added to the reentrant angular corner.
The intent of the method described in this patent document is to have the first machining pass remove the rounded portions generated in reentrant angular corners, which cause a problem in the machining of dies or the like, by adding a circular arc machining route having a radius equal to the offset on the workpiece side, centered at the block intersection point. This circular arc route is used to gouge the workpiece in the first machining pass and is not designed to increase the machining accuracy in the corner. Accordingly, the technique shown in this patent document addresses a different problem from that of the present invention, which is described below.
(2) Japanese Patent Application Laid-Open No. 2002-011620 discloses an attempt to machine a workpiece into a desired shape by changing the machining conditions at appropriate points in a wire cut electrical discharge machining process that allows the machining conditions to be changed in ways that cause variations in the discharging gap of the wire electrode. According to this patent document, if the offset is changed at the point at which two blocks are connected, a correction block is inserted at the point at which the machining route becomes discontinuous and an appropriate timing is set at which to change the machining conditions. The technique shown in this patent document does not address the problem of the increased machining depth due to the offset in reentrant angular corners and is again different from the technique of the present invention.
(3) Japanese Patent Application Laid-Open No. 2004-148472 discloses a method for changing the feed speed at appropriate points in the finishing stage of reentrant angular corners to solve the above problem. More specifically, in the finishing stage, the feed speed is changed at four points: a first point at which the machining removal distance in an offset path offset from a predetermined path starts to increase or decrease in relation to the machining removal distance in a straight line; a second point at which the machining removal distance ceases to change after the wire electrode has passed the first point; a third point at which the machining removal distance starts to decrease or increase after the wire electrode has passed the second point; and a fourth point at which the machining removal distance becomes equal to the distance removed in machining a straight line after the wire electrode has passed the third point. Then, the machining removal distance is calculated at intervals of a predetermined unit distance between the first point and the fourth point, and an appropriate feed speed is determined from each calculated machining removal distance, so that the feed speed is appropriately changed for each unit distance between the first point and the fourth point.
The technique disclosed in this patent document addresses the same problem as the present invention, but attempts to solve the problem by controlling the machining speed, so it is different from the solution provided by the present invention.
(4) Japanese Patent Application Laid-Open No. H04-217426 discloses a technique for improving the shape accuracy of a corner which is specified as a circular arc route in a machining program. More specifically, as the machining process proceeds from rough machining to finish machining, if a circular arc in the shape to be machined has a radius smaller than a preset reference radius, the circular section is first machined along a route in which arcs with a radius smaller than the intended radius are inserted, tangent to the machining routes at both ends of the intended arc, and then the machining route is modified by gradually increasing the radius of the arcs to be inserted.
The technique disclosed in this patent document attempts to minimize the machining depth in the finishing stage by cutting as deeply as possible into the corner in the rough machining stage, so it addresses a problem different from that of the present invention, which is directed toward the improvement of machining accuracy in reentrant angular corners (i.e., corners having a reentrant angular route in the machining program). The technique in this patent document looks similar to the present invention regarding the insertion of a circular arc route, but differs completely from the present invention in the way the circular arc machining route is determined. The present invention determines the circular arc machining route from the offset difference as described later.
(5) Japanese Patent Application Laid-Open No. 07-009261 discloses another technique for improving the shape accuracy of a corner which is specified as a circular arc route in a machining program. More specifically, the technique disclosed in this patent document attempts to minimize the minimum machinable radius in reentrant angular corners by creating machining routes with the same radius in each machining step. To do this, the locus of movement in inside corners in each machining step is calculated so that the circular arc loci produce the same radius in inside-corner machining in machining steps with different offsets, and the locus of movement is controlled based on the calculation result so that the inside-corner radius becomes equal in each machining step.
In contrast, the present invention attempts to improve the shape accuracy of corners that are specified as reentrant angular routes in the machining program and the circular arc machining route to be inserted has a different radius in each machining pass. In the technique described in this patent document, as can be seen from FIG. 2, the machining margin is clearly greater in a reentrant angular corner than in a straight section. Unlike the above technique, the present invention attempts to make the machining margin in reentrant angular corners equal to that in straight sections, as described later.